Sensory neurobiology: Muscles power pheromone sensation.

While we understand how the five main sensory organs enable and facilitate stimulus detection, little is known about how the vomeronasal organ enables pheromone sensation. A new study finds specialized muscles poised to coordinate stimulus delivery, dynamics, and arousal.

Less is more: Hormonal-induced decrease in brain activity is required for associative learning.

Memory develops during early life, yet the corresponding molecular mechanisms are largely unknown. Leinwand and Scott (2021) reveal a link between juvenile hormone, neural activity, and memory-evoked behavior during a critical period that promotes associative learning in the adult fly.

Flexible scaling and persistence of social vocal communication.

Innate vocal sounds such as laughing, screaming or crying convey one's feelings to others. In many species, including humans, scaling the amplitude and duration of vocalizations is essential for effective social communication1-3. In mice, female scent triggers male mice to emit innate courtship ultrasonic vocalizations (USVs)4,5. However, whether mice flexibly scale their vocalizations and how neural circuits are structured to generate flexibility remain largely unknown. Here we identify mouse neurons from the lateral preoptic area (LPOA) that express oestrogen receptor 1 (LPOAESR1 neurons) and, when activated, elicit the complete repertoire of USV syllables emitted during natural courtship. Neural anatomy and functional data reveal a two-step, di-synaptic circuit motif in which primary long-range inhibitory LPOAESR1 neurons relieve a clamp of local periaqueductal grey (PAG) inhibition, enabling excitatory PAG USV-gating neurons to trigger vocalizations. We find that social context shapes a wide range of USV amplitudes and bout durations. This variability is absent when PAG neurons are stimulated directly; PAG-evoked vocalizations are time-locked to neural activity and stereotypically loud. By contrast, increasing the activity of LPOAESR1 neurons scales the amplitude of vocalizations, and delaying the recovery of the inhibition clamp prolongs USV bouts. Thus, the LPOA disinhibition motif contributes to flexible loudness and the duration and persistence of bouts, which are key aspects of effective vocal social communication.

PIEZO2 in sensory neurons and urothelial cells coordinates urination.

Henry Miller stated that "to relieve a full bladder is one of the great human joys". Urination is critically important in health and ailments of the lower urinary tract cause high pathological burden. Although there have been advances in understanding the central circuitry in the brain that facilitates urination1-3, there is a lack of in-depth mechanistic insight into the process. In addition to central control, micturition reflexes that govern urination are all initiated by peripheral mechanical stimuli such as bladder stretch and urethral flow4. The mechanotransduction molecules and cell types that function as the primary stretch and pressure detectors in the urinary tract mostly remain unknown. Here we identify expression of the mechanosensitive ion channel PIEZO2 in lower urinary tract tissues, where it is required for low-threshold bladder-stretch sensing and urethral micturition reflexes. We show that PIEZO2 acts as a sensor in both the bladder urothelium and innervating sensory neurons. Humans and mice lacking functional PIEZO2 have impaired bladder control, and humans lacking functional PIEZO2 report deficient bladder-filling sensation. This study identifies PIEZO2 as a key mechanosensor in urinary function. These findings set the foundation for future work to identify the interactions between urothelial cells and sensory neurons that control urination.

Representation of Olfactory Information in Organized Active Neural Ensembles in the Hypothalamus.

The internal representation of sensory information via coherent activation of specific pathways in the nervous system is key to appropriate behavioral responses. Little is known about how chemical stimuli that elicit instinctive behaviors lead to organized patterns of activity in the hypothalamus. Here, we study how a wide range of chemosignals form a discernible map of olfactory information in the ventromedial nucleus of the hypothalamus (VMH) and show that different stimuli entail distinct active neural ensembles. Importantly, we demonstrate that this map depends on functional inputs from the vomeronasal organ. We present evidence that the spatial locations of active VMH ensembles are correlated with activation of distinct vomeronasal receptors and that disjunct VMH ensembles exhibit differential projection patterns. Moreover, active ensembles with distinct spatial locations are not necessarily associated with different behavior categories, such as defensive or social, calling for a revision of the currently accepted model of VMH organization.

Bespoke behavior: mechanisms that modulate pheromone-triggered behavior.

What is good for others, may not be in my best interest. Individuals should not, and do not, respond identically in the same environment. Personalized social behavior is particularly important to ultimately ensure reproductive fitness. How and where neural activity is modulated to customize behavior has remained largely unknown. The robust response to pheromones provides a platform to identify the logic of how the brain initiates social behavior. Mouse pheromones engage innate motor actions that underlie social behavior yet are plastic to suit individual needs. Recent study of mouse pheromone behavior, neurocircuit activity, and functional manipulations is beginning to paint a complex, dynamic, and diverse picture of the mechanisms that enable flexible modulation of social behavior.

Choosing to urinate. Circuits and mechanisms underlying voluntary urination.

The decision to urinate is a social behavior that is calculated multiple times a day. Many animals perform urine scent-marking which broadcasts their pheromones to regulate the behavior of others and humans are trained at an early age to urinate only at a socially acceptable time and place. The inability to control when and where to void, that is incontinence, causes extreme social discomfort yet targeted therapeutics are lacking because little is known about the underlying circuits and mechanisms. The use of animal models, neurocircuit analysis, and functional manipulation is beginning to reveal basic logic of the circuit that modulates the decision of when and where to void.

Animal Behavior: Honesty Can Kill.

Virgin male mice naturally kill another male's pups so they can sire their own offspring. New research shows that pups are identified using a combination of generic and 'honest' cues, revealing an unexpected logic underlying pup recognition and ensuing infanticide.

Voluntary urination control by brainstem neurons that relax the urethral sphincter.

Voluntary urination ensures that waste is eliminated when safe and socially appropriate, even without a pressing urge. Uncontrolled urination, or incontinence, is a common problem with few treatment options. Normal urine release requires a small region in the brainstem known as Barrington's nucleus (Bar), but specific neurons that relax the urethral sphincter and enable urine flow are unknown. Here we identify a small subset of Bar neurons that control the urethral sphincter in mice. These excitatory neurons express estrogen receptor 1 (BarESR1), project to sphincter-relaxing interneurons in the spinal cord and are active during natural urination. Optogenetic stimulation of BarESR1 neurons rapidly initiates sphincter bursting and efficient voiding in anesthetized and behaving animals. Conversely, optogenetic and chemogenetic inhibition reveals their necessity in motivated urination behavior. The identification of these cells provides an expanded model for the control of urination and its dysfunction.

Social Behavior: How the Brain Thinks like a Mom.

Becoming a parent changes our choices and actions. Identifying the underlying neural circuits is necessary to understand the transformation of an animal's behavior post-parenthood. Multiple nodes of the 'parenting circuit' have now been identified to reveal the workings of a single brain region key to the orchestration of parent-specific behaviors.

Bmal1 Is Required for Normal Reproductive Behaviors in Male Mice.

Circadian rhythms synchronize physiological processes with the light-dark cycle and are regulated by a hierarchical system initiated in the suprachiasmatic nucleus, a hypothalamic region that receives direct photic input. The suprachiasmatic nucleus then entrains additional oscillators in the periphery. Circadian rhythms are maintained by a molecular transcriptional feedback loop, of which brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein 1 (BMAL1) is a key member. Disruption of circadian rhythms by deletion of the BMAL1 gene (Bmal1 knockout [KO]) induces a variety of disease states, including infertility in males, due to unidentified mechanisms. We find that, despite normal sperm function, Bmal1 KO males fail to mate with receptive females, indicating a behavioral defect. Mating is dependent on pheromone detection, as are several other behaviors. We determined that Bmal1 KO males also fail to display aggression and avoidance of predator scent, despite intact main olfactory function. Moreover, the vomeronasal organ, a specialized pheromone-responsive organ, was also functionally intact, as determined by calcium imaging in response to urine pheromone stimulus. However, neural circuit tracing using c-FOS activation revealed that, although Bmal1 KO males displayed appropriate activation in the olfactory bulb and accessory olfactory bulb, the bed nucleus of the stria terminalis and the medial preoptic area (areas responsible for integration of copulatory behaviors) failed to activate highly in response to the female scent. This indicates that neural signaling in select behavioral centers is impaired in the absence of BMAL1, likely underlying Bmal1 KO male copulatory defects, demonstrating the importance of the BMAL1 protein in the maintenance of neural circuits that drive pheromone-mediated mating behaviors.

Think You Know How Smell Works? Sniff Again.

The sense of smell is mediated by GPCRs in the odorant receptor (OR) family. Greer et al. report a new family of odor detectors, MS4As, that have similar cellular localization and chemodetection ability as ORs but are not GPCRs and follow a strikingly different logic of odor coding at the periphery.

State-dependent responses to sex pheromones in mouse.

A single sensory cue can evoke different behaviors that vary by recipient. Responses may be influenced by sex, internal state, experience, genotype, and coincident environmental stimuli. Pheromones are powerful inducers of mouse behavior, yet pheromone responses are not always stereotyped. For example, male and female mice respond differently to sex pheromones while mothers and virgin females respond differently to pup cues. Here, we review the origins of variability in responses to reproductive pheromones. Recent advances have indicated how response variability may arise through modulation at different levels of pheromone-processing circuitry, from sensory neurons in the periphery to central neurons in the vomeronasal amygdala. Understanding mechanisms underlying conditional pheromone responses should reveal how neural circuits can be flexibly sculpted to alter behavior.

Cyclic Regulation of Sensory Perception by a Female Hormone Alters Behavior.

Females may display dramatically different behavior depending on their state of ovulation. This is thought to occur through sex-specific hormones acting on behavioral centers in the brain. Whether incoming sensory activity also differs across the ovulation cycle to alter behavior has not been investigated. Here, we show that female mouse vomeronasal sensory neurons (VSNs) are temporarily and specifically rendered "blind" to a subset of male-emitted pheromone ligands during diestrus yet fully detect and respond to the same ligands during estrus. VSN silencing occurs through the action of the female sex-steroid progesterone. Not all VSNs are targeted for silencing; those detecting cat ligands remain continuously active irrespective of the estrous state. We identify the signaling components that account for the capacity of progesterone to target specific subsets of male-pheromone responsive neurons for inactivation. These findings indicate that internal physiology can selectively and directly modulate sensory input to produce state-specific behavior. PAPERCLIP.

Mammalian pheromones: emerging properties and mechanisms of detection.

The concept of mammalian pheromones was established decades before the discovery of any bioactive ligands. Therefore, their molecular identity, native sources, and the meaning of their detection has been largely speculative. There has been recent success in identifying a variety of candidate mouse pheromones and other specialized odors. These discoveries reveal that mammalian pheromones come in a variety of ligand types and they are detected by sensory neurons that are pre-set to promote an array of social and survival behaviors. Importantly, recent findings show that they activate molecularly diverse sensory neurons that differ from canonical odorant detectors. These novel sensory neurons hold future promise to unlock the mystery of how their detection is hardwired to generate behavior.

Murine pheromone proteins constitute a context-dependent combinatorial code governing multiple social behaviors.

During social interactions, an individual's behavior is largely governed by the subset of signals emitted by others. Discrimination of "self" from "other" regulates the territorial urine countermarking behavior of mice. To identify the cues for this social discrimination and understand how they are interpreted, we designed an olfactory-dependent countermarking assay. We find major urinary proteins (MUPs) sufficient to elicit countermarking, and unlike other vomeronasal ligands that are detected by specifically tuned sensory neurons, MUPs are detected by a combinatorial strategy. A chemosensory signature of "self" that modulates behavior is developed via experience through exposure to a repertoire of MUPs. In contrast, aggression can be elicited by MUPs in an experience-independent but context-dependent manner. These findings reveal that individually emitted chemical cues can be interpreted based on their combinatorial permutation and relative ratios, and they can transmit both fixed and learned information to promote multiple behaviors.

Live cell calcium imaging of dissociated vomeronasal neurons.

Sensory neurons in the vomeronasal organ (VNO) are thought to mediate a specialized olfactory response. Currently, very little is known about the identity of stimulating ligands or their cognate receptors that initiate neural activation. Each sensory neuron is thought to express 1 of approximately 250 variants of Vmn1Rs, Vmn2Rs (A, B, or D), or FPRs which enables it to be tuned to a subset of ligands (Touhara and Vosshall, Annu Rev Physiol 71:307-332, 2009). The logic of how different sources of native odors or purified ligands are detected by this complex sensory repertoire remains mostly unknown. Here, we describe a method to compare and analyze the response of VNO sensory neurons to multiple stimuli using conventional calcium imaging. This method differs from other olfactory imaging approaches in that we dissociate the tightly packed sensory epithelium into individual single cells. The advantages of this approach include (1) the use of a relatively simple approach and inexpensive microscopy, (2) comparative analysis of several hundreds of neurons to multiple stimuli with single-cell resolution, and (3) the possibility of isolating single cells of interest to further analyze by molecular biology techniques including in situ RNA hybridization, immunofluorescence, or creating single-cell cDNA libraries (Malnic et al., Cell 96:713-723, 1999).

Ominous odors: olfactory control of instinctive fear and aggression in mice.

Aggression and fear are often thought to be distinct behavioral states, yet they share several common output responses. In the mouse, both can be initiated by specialized odor cues. How these cues signal through the olfactory system to promote behavior is largely unknown. Recent experiments have started to uncover the relevant signaling ligands, chemosensory receptors, and responsive sensory neurons that together enable the precise manipulation of behaviorally relevant neural circuits. Moreover, the use of molecular genetics and new experimental strategies has begun to reveal how the central nervous system processes olfactory information to initiate aggression and fear. A sensory-initiated comparative study of these two fundamental threat reactions promises to offer new mechanistic insight.

Synchronous evolution of an odor biosynthesis pathway and behavioral response.

BACKGROUND: Rodents use olfactory cues for species-specific behaviors. For example, mice emit odors to attract mates of the same species, but not competitors of closely related species. This implies rapid evolution of olfactory signaling, although odors and chemosensory receptors involved are unknown. RESULTS: Here, we identify a mouse chemosignal, trimethylamine, and its olfactory receptor, trace amine-associated receptor 5 (TAAR5), to be involved in species-specific social communication. Abundant (>1,000-fold increased) and sex-dependent trimethylamine production arose de novo along the Mus lineage after divergence from Mus caroli. The two-step trimethylamine biosynthesis pathway involves synergy between commensal microflora and a sex-dependent liver enzyme, flavin-containing monooxygenase 3 (FMO3), which oxidizes trimethylamine. One key evolutionary alteration in this pathway is the recent acquisition in Mus of male-specific Fmo3 gene repression. Coincident with its evolving biosynthesis, trimethylamine evokes species-specific behaviors, attracting mice, but repelling rats. Attraction to trimethylamine is abolished in TAAR5 knockout mice, and furthermore, attraction to mouse scent is impaired by enzymatic depletion of trimethylamine or TAAR5 knockout. CONCLUSIONS: TAAR5 is an evolutionarily conserved olfactory receptor required for a species-specific behavior. Synchronized changes in odor biosynthesis pathways and odor-evoked behaviors could ensure species-appropriate social interactions.

Learned recognition of maternal signature odors mediates the first suckling episode in mice.

BACKGROUND: Soon after birth, all mammals must initiate milk suckling to survive. In rodents, this innate behavior is critically dependent on uncharacterized maternally derived chemosensory ligands. Recently, the first pheromone sufficient to initiate suckling was isolated from the rabbit. Identification of the olfactory cues that trigger first suckling in the mouse would provide the means to determine the neural mechanisms that generate innate behavior. RESULTS: Here we use behavioral analysis, metabolomics, and calcium imaging of primary sensory neurons and find no evidence of ligands with intrinsic bioactivity, such as pheromones, acting to promote first suckling in the mouse. Instead, we find that the initiation of suckling is dependent on variable blends of maternal "signature odors" that are learned and recognized prior to first suckling. CONCLUSIONS: As observed with pheromone-mediated behavior, the response to signature odors releases innate behavior. However, this mechanism tolerates variability in both the signaling ligands and sensory neurons, which may maximize the probability that this first essential behavior is successfully initiated. These results suggest that mammalian species have evolved multiple strategies to ensure the onset of this critical behavior.